1. Field of the Invention
This invention relates to a multilayer printed wiring board and manufacturing process therefor, and particularly to a multilayer printed wiring board with a plurality of metal wiring layers and insulation layers laminated on the board and manufacturing process therefor.
2. Description of the Prior Art
In conventional multilayer printed wiring board, multiple thin film layers are structured by alternatively laminating insulation layers (made of polyimide resin, for example) and metal wiring layers on a ceramic base substrate as the foundation. Further, in the manufacturing process for such a multilayer printed wiring board, lamination of insulation and metal wiring layers is started from the lowest layer and a series of steps is repeated until the required number of layers are completed.
Such conventional manufacturing methods where a multilayer printed wiring board is fabricated by repetition of conductor and insulation layers laminating on the board include the method disclosed in "Copper/polyimide Materials System for High Performance Packaging" by R. J. Jenson and J. P. Cummings and H. Vora (1984, IEEE) and the one disclosed in U.S. Pat. No. 4,434,544.
In case of such conventional multilayer printed wiring board, since ceramic base substrate and polyimide used for insulation layer have different thermal expansion coefficients, thermal stress is generated at the base substrate during forming of multiple thin film layers for wiring, which results in residual stress between insulation layer and base substrate. This residual stress may lead to troubles such as cracks on polyimide resin, separation from ceramic multilayer printed wiring board and fracture at ceramic multilayer printed wiring board. In addition, with recent progress toward higher density mounting, film thickness of the multiple thin film layers for wiring consisting of polyimide resin insulation layer and metal wiring layer tends to be thicker. The thicker the film thickness becomes, the larger the above residual stress grows and the above troubles such as cracks on polyimide resin and separation and fracture at ceramic multilayer printed wiring board are becoming more serious.
Besides, in the conventional process to manufacture multilayer printed wiring board, manufacturing starts at the lowest layer first and a series of steps is repeated for lamination of insulation and metal wiring layers until the required number of layers are completed. This method takes quite a long time when a large number of thin film layers for wiring have to be formed on the ceramic base substrate. In addition, there is a possibility of worse yield, because unexpected defects may occur during too long manufacturing time.